1. Field of the Invention
This invention relates generally to a torque converter for an automatic transmission, and, in particular, to a hydraulic system that supplies oil to the converter, actuates an impeller clutch of the torque converter and provides a continuous supply of hydraulic lubricant to transmission components.
2. Description of the Prior Art
A torque converter is a modified form of a hydrodynamic fluid coupling, and like a fluid coupling, is used to transfer rotating power from a prime mover, such as an internal combustion engine or electric motor, to a rotating driven load. A torque converter is able to multiply torque when there is a substantial difference between input and output rotational speed, thus providing the equivalent of a reduction gear.
A torque converter includes at least three rotating elements: an impeller, which is mechanically driven by the prime mover; a turbine, which drives the load; and a stator, which is interposed between the impeller and turbine so that it can alter oil flow returning from the turbine to the impeller to multiply torque. The stator is mounted on an overrunning clutch, which prevents the stator from counter-rotating the prime mover but allows for forward rotation. The torque converter is encased in a housing, which contains automatic transmission fluid (ATF), sometimes referred to as “oil,” “lube” or “lubricant.”
Hydrodynamic parasitic losses within the torque converter reduce efficiency and generate waste heat. In modern automotive applications, this problem is commonly avoided by use of a bypass clutch (also called lock-up clutch), which physically links the impeller and turbine, effectively changing the converter into a purely mechanical coupling. The result is no slippage, virtually no power loss and improved fuel economy.
Torque converter clutch designs include two basic types, a closed piston design and an open piston design. A closed piston design requires a dedicated hydraulic circuit into the torque converter, which communicates only with the apply side of the clutch piston. When pressure is high, the clutch applies. When pressure is low, the clutch releases. A more uncommon form is to have this circuit on the release side where high pressure releases the clutch and low pressure applies the clutch.
When the torque converter is multiplying torque, power loss occurs which significantly increases the temperature of ATF in the torque converter and must be cooled before returning to the transmission. Cooler return oil is usually routed into the transmission lubrication circuit to cool internal clutches, gear sets and bearings. The lubrication circuit is also used to fill or charge balance dams, which are intended to keep disengaged clutch pistons from drifting on when internal rotational speeds increase.
Hydraulic system logic that controls a torque converter is responsible for several functions including: 1) supplying the converter with sufficient pressure to keep the converter from cavitating, 2) flowing sufficient oil through the converter to remove heat generated in the torus and clutch, 3) controlling pressure in the lock-up clutch piston, 4) supplying oil to the cooling and lube circuits, and 5) minimizing system pump demand for flow and pressure when not required. Many controls systems do not properly control all of these functions
A need in the industry exists to control a closed piston torque converter bypass clutch using a simple valve arrangement, that improves clutch control, reduces converter flow demands without introducing risk to the transmission lubrication system.